The present invention relates to a copolyester having a high transparency, superior strength and superior gas barrier property, and a packaging material produced by the copolyester.
Polyethylene terephthalate (hereinafter, sometimes abbreviated as "PET") is excellent in mechanical strength, chemical safety, transparency, and hygienic property and is light in weight and low in cost. Therefore PET is widely used in packaging materials such as sheets, films and containers.
However, although the gas barrier property of PET is superior comparing with other resins including polyolefin, it is not considered to be sufficient in some application fields where a higher performance is required. For instance, a packaging material for carbonated beverage, beer and wine is required to have high oxygen gas barrier property and carbon dioxide gas barrier property in order to preserve the content, and a generally used hollow container of a biaxially oriented PET cannot satisfy these strict requirements on gas barrier properties. Moreover, there is a demand to supply a film which has low permeability of gas such as oxygen or moisture, and does not lower its performance under freezing process, boiling treatment and retort treatment, for the packaging of fresh food, medical equipment etc.
To solve these problems, various methods for improving the gas barrier property of packaging materials prepared from polyethylene terephthalate, for example, a method wherein various gas barrier materials are blended with PET and a method wherein various gas barrier materials are laminated or coated on PET film, have been proposed.
The present inventors have found that a specific copolyester obtained by using isophthalic acid and phenylenedioxy diacetic acid as the main acid components can be used as superior gas barrier material (Japanese Patent Application Laid-Open (KOKAI) Nos. 1-247422(1989), 21-47423(1989), 1-167331(1989), 2-182455(1990) and 2-14238(1990), and U.S. Pat. No. 4,959,421).
Also, it is known that the polyester copolymer prepared by 5 to 100 mol % of phenylenedioxy diacetic acid and 0 to 95 mol % of terephthalic acid as acid component is an excellent material in view of gas barrier property (Japanese Patent Application Laid-Open (KOKAI) No. 60-501060(1985), U.S. Pat. No. 4,440,992, and U.S. Pat. No. 4,552,948).
However, generally these gas barrier materials do not show sufficient mechanical strength and heat resistance so that it is difficult to mold these materials singly. When a gas barrier material is blended to PET, it is difficult to maintain the mechanical strength, heat resistance, etc., which are inherent in PET. Moreover, on the method of coating or laminating a gas barrier material onto PET, there are various problems to be considered in the selection of molding condition, adhesive property between layers, or thickening of container wall.